Preheat the oven to 190°C. In a large pan, dry fry the beef and onion for 4 to 5 minutes until browned. Stir in the herbs, flour, stock and tomato puree, and season. Bring to the boil and simmer for 30 minutes until thickened. Transfer the beef mixture to an ovenproof gratin dish. Cover with a layer of the sliced tomatoes and then add a layer of sliced courgettes. Set aside until required. Blend the cornflour with a little milk in a small bowl. Pour the remaining milk into a saucepan and bring to the boil. Add the cornflour mixture and cook, stirring for 1 to 2 minutes until thickened. Remove from the heat and beat in the fromage frais and egg yolk. Season well. Place the dish on to a baking sheet and spread the white sauce over the layer of courgettes. Sprinkle with grated Parmesan and bake in the oven for 25 to 30 minutes until golden brown. Serve immediately.